Signal measurements of serving and neighboring cells are important for optimized mobility. The LTE standard supports variable bandwidth (BW)—from 1.4 MHz to 20 MHz—but mobility performance must be the same regardless of system BW. It is known for a communication network to signal one common measurement bandwidth, for use by a User Equipment (UE) in performing certain mobility measurements (e.g. Received Signal Received Power, “RSRP”, and/or Received Signal Received Quality, “RSRQ”, measurements) in Radio Resource Control (RRC) IDLE and CONNECTED states.
Thus, in an example where different cells on the same carrier frequency operate over 5 MHz and 10 MHz bandwidths, for example, then the network can request that the UE measure over 5 MHz (i.e., a common measurement BW for all cells). However in early releases of the LTE standard, the minimum mobility measurement requirements (i.e., the RSRP/RSRQ accuracy) are to be met by a UE measuring signal quality of a downlink carrier from a cell in the network on six or more of the Resource Blocks (RBs) at the center of the downlink carrier bandwidth. This approach allows the same minimum mobility measurement performance regardless of whether the downlink carrier bandwidth is 1.4 MHz or 20 MHz.
Now, it may be that all cells on the same downlink carrier frequency are configured with the same bandwidth of, say, 10 MHz. In such a scenario, reasonable signal quality estimation may be obtained by the UE measuring signal quality on the six central RBs of each downlink carrier of interest, because the load is typically evenly spread over the full bandwidth of the downlink carrier. However there are advanced deployment network deployment scenarios where such signal quality approaches may not work well.
For example, in cases where a network operator is migrating from one radio access technology to another (say, WCDMA to LTE) the migration will typically be carried out gradually. Thus, it may be that one or more areas in the network will use large bandwidth cells, e.g., LTE carrier bandwidths of 10 MHz, while neighbouring cells overlapping the large-bandwidth cells will use smaller-bandwidth carriers, e.g., 5 MHz LTE carriers, and/or 5 MHz WCDMA carriers.
In another example, the network operator may be licensed for a 10 MHz bandwidth and may use that full bandwidth in a cell associated with a hotspot where network usage is high, e.g., an urban area with many potential simultaneous users. However, other cells in the same network may use only half of the bandwidth. Such reduced-bandwidth cells may overlap the hotspot cell or cells, but not each other, in order to reduce interference, and hence allow for lower transmission power (or alternatively, for wider coverage). Such a configuration may also be combined with carrier aggregation at (rural) hot spot areas. There also may be two network operators having licenses for say 5 MHz each, and at particular hotspot areas they combine their bandwidths to offer coverage in one or more 10 MHz cells.
In yet another scenario, an operator may have a license for a particular bandwidth, say 10 MHz, but uses a variable bandwidth in dependence on load conditions in order to save power (e.g., “green base stations”). During times with high traffic volumes, cells operate with the full bandwidth, but at times with low volumes, the cells are re-configured and re-arranged to have smaller bandwidths and to be distributed within the band such that some neighbor cells are on different carriers (reduction of interference). Cells with high load—e.g., in particular hotspot areas—may still have the full bandwidth.
In yet another scenario an operator may operate cells in heterogeneous network using some mix of full-bandwidth cells (e.g., 10 MHz) and partial-bandwidth cells. For example, in a two-tier heterogeneous network deployment scenario that includes high power nodes and low power nodes (e.g., a mix of macro-pico cells, macro-femto cells, macro-CSG cells, etc.), the high power nodes and lower power nodes may operate over 5 MHz and 10 MHz bandwidths, respectively. In an example network, a number of macro base stations provide corresponding macro cells, and a number of pico or femto base stations provide corresponding pico cells among or overlaid with one or more of the macro cells.
Yet another scenario involves coordinated multi-point transmission and reception (CoMP operation), which is also known as a multi-point communication system. With CoMP, a transmission originates at different locations—e.g. from non co-located BSs or from a combination of BSs and Remote Radio Heads (RRHs) and/or Remote Radio Units (RRUs). Other well-known examples of multi-point communication are DAS, RRH, RRU, etc. In such scenarios, the UE operates over more than one radio links—i.e., it receives from and/or transmits towards different locations. The network may use different bandwidths over different radio links. CoMP also may be used in conjunction with carrier aggregation.
In yet another scenario, all cells on a given downlink carrier frequency may have the same bandwidth (e.g. 10 MHz). However, the network may schedule downlink data to the UE from different cells in an orthogonal manner. For example in neighboring cells 1, 2 and 3, the network may schedule RBs over lower, central and upper parts of the bandwidth. However, the assigned resources also may change over time because of the lack of traffic or because of varying traffic situations in the involved cells.
Notably, signal quality measurements made on the downlink by a UE (e.g., RSRQ, RSSI or other measurements) are primarily used for mobility-related radio operation tasks, in both idle and connected states. In idle mode, the mobility-related radio operations include cell selection and cell reselection, including intra-frequency, inter-frequency and inter-RAT (e.g., between UTRA and LTE). In the connected state, mobility-related radio operations include cell change, handover, RRC connection re-establishment, RRC connection release with direction to a target cell, and primary component carrier, “PCC”, change in carrier aggregation operation, or primary cell, “PCell”, change in carrier aggregation.
RSRQ and other quality measurements are also used for radio operation tasks beyond those related to mobility, such as enhanced cell ID positioning, fingerprinting positioning, minimization of drive tests (MDT), network planning, configuration and tuning of radio network parameters, self organizing network (SON) operations, network monitoring, etc. Such radio operations involve, for example, the UE making and reporting signal quality determinations to the network, for one or more downlink carriers.
The mix of carrier bandwidths and/or center frequencies poses certain challenges with respect to accurate determination of signal quality by a UE. Signal quality determination may also be affected by the use of “spectrum margins”, wherein the real bandwidth of downlink carriers is somewhat less than the nominal full bandwidth, to guard against spectral leakage at the bandwidth boundaries.